Youngchul Chung

An assessment of finite difference beam propagation method

A finite-difference beam propagation method (FD-BPM) is outlined and assessed in comparison with a conventional beam propagation method (FFT-BPM) which uses fast Fourier transformation. In the comparative study three straight waveguides with different index profiles that are frequently encountered in integrated optics are utilized. Using both methods normalized effective index values of the eigenmodes of these waveguides are calculated and compared with the exact values obtained from analytical expressions. As a further accuracy criterion, the power loss due to numerical errors when an eigenmode of a waveguide is excited is evaluated. Based on this comparison the accuracy, computational efficiency, and stability of the FD-BPM are assessed. >

Modal reflection of quarter-wave mirrors in vertical-cavity lasers

Very high plane-wave reflection coefficients can be obtained with practical semiconductor quarter-wave mirrors, but for beams of finite width, the reflection coefficient of a mirror with no lateral guiding and hence the finesse of cavities that use such structures will be limited by diffraction loss. The authors analytically and numerically study the modal reflection of practical semiconductor quarter-wave mirrors. They introduce a quantity called the diffraction range of a quarter-wave mirror as a means of exact analytical comparison between infinite lossless mirrors (and approximate comparison for finite mirrors) in the Fresnel diffraction limit. The exact modal reflection coefficient for an arbitrary incident mode pattern is determined by vector plane-wave decomposition. The modal reflection coefficients of two representative semiconductor quarter-wave mirrors used in vertical cavity laser technology, AlAs/GaAs and InGaAsP/InP, are studied. >

Analysis of Z-invariant and Z-variant semiconductor rib waveguides by explicit finite difference beam propagation method with nonuniform mesh configuration

An efficient and simple explicit finite difference beam propagation method (EFD-BPM) incorporating nonuniform mesh is described. The criteria for stability are developed, and it is shown that this algorithm is power conserving when the stability criteria are met. EFD-BPM is applied to the analysis of single and coupled semiconductor rib waveguides and its accuracy is confirmed by comparing the results with the reported results. Nonuniform mesh is found to improve the efficiency of the method significantly for the analysis of weakly guiding waveguide structures. Several coupled rib waveguide structures with curved input and output branching sections are analyzed using both three-dimensional EFD-BPM and two-dimensional finite difference BPM combined with effective index approximation. >

An efficient split-step time-domain dynamic modeling of DFB/DBR laser diodes

A novel and efficient approach for the numerical solution of time-dependent coupled-wave equations, which are frequently used for the modeling of distributed-feedback, distributed Bragg reflector, and Fabry-Perot laser diodes, is proposed. In this approach, the coupled-wave equations are split into two sets of equations. One of two sets of equations contains only the phase factors and time derivatives, and the other contains only the coupling terms. The separate sets of equations are solved exactly in their split form successively. This new numerical scheme, which we call the split-step time-domain model, is found to require an order of magnitude smaller number of subsections to get more accurate results than previous methods while the computation time for each time step is comparable to previous methods.

On leaky mode approximations for modal expansion in multilayer open waveguides

We propose an analytic method to calculate the leaky mode functions to enable modal expansion without encountering the normalization and orthogonality problems of the unphysical mode shape. The wave functions of leaky modes are derived from the explicit formulas of normalized radiation modes. Using leaky modes to approximate a continuum of radiation modes greatly simplifies the analysis of excitation, transitions, propagation, and radiation of light waves in multilayer waveguide structures. Upon comparison with the beam propagation method and modal propagation method, we show that the leaky mode approximation while requiring a much simpler computation is as accurate. This method can also provide better physical insight to device operations. >

New design for low-loss star couplers and arrayed waveguide grating devices

We propose a new loss reduction method in star couplers employing UV-written tapers that replace the free propagation region in the conventional star couplers and apply them to an arrayed waveguide grating (AWG) device. The insertion loss of the new AWG device can be reduced to 0.31 dB, which is about 0.7 dB lower than that of the AWG without UV-written tapered waveguides.

Total internal reflection mirror-based InGaAsP ring resonators integrated with optical amplifiers

Novel ring resonators combining very small multimode interference (MMI) couplers, low loss total internal reflection (TIR) mirrors, and a semiconductor optical amplifier in InGaAsP material system are reported for the first time. The MMI length of 113 /spl mu/m is among the shortest reported. Average TIR mirror loss is about 1.1 dB per mirror. The material platform and fabrication process used are the same used for other active and passive devices except for a deep etch step. Hence, such resonators are easily integrated with other active and passive devices. A free spectral range of approximately 2 nm is observed near 1568 nm along with an on-off ratio of 14 dB, a full-width at half-maximum of about 0.3 nm, a finesse of more than 6, and a Q-factor of more than 4900.

Field-induced waveguides and their application to modulators

The fabrication and characterization of field-induced waveguides (FIGs) as well as guide/antiguide modulators which utilize the FIG concept are presented. Both theoretical and experimental results promise that a relatively strongly confined, low-loss optical waveguide can be achieved. By applying the appropriate bias to the modulator, a guiding or antiguiding situation can be created which corresponds to on- and off-states, respectively. An optical bandwidth from 1 to 1.55 mu m, and a propagation loss of 1 dB at 1.3 mu m has been achieved. >

Analysis of integrated optical corner reflectors using a finite-difference beam propagation method

Integrated optical corner reflectors in III-V semiconductors are analyzed employing a finite-difference beam propagation method and propagating the beam in parallel with the etched semiconductor-air interface. For this choice of propagation direction, the effects of mirror roughness, rotation, and displacement of the mirror surface from its ideal position can be assessed very easily. The integrated reflector whose mode size is larger shows less dependence on the mirror displacement error. The loss due to mirror surface roughness depends weakly on the mode size and strongly on the mode polarization, being larger for the quasi-transverse-electric polarization. The loss due to rotational errors of the mirror surface is not a strong function of polarization, but increases as the waveguide width increases. However, for a rotation error smaller than 0.1 degrees , which should be achieved easily, the excess loss is smaller than 0.2 dB at 1.3 % mu m regardless of the waveguide width.<<ETX>>

Design and analysis of widely tunable sampled grating DFB laser diode integrated with sampled grating distributed Bragg reflector

A new widely tunable laser diode structure that requires only two tuning currents is proposed. The laser diode consists of a sampled grating distributed feedback (SGDFB) laser diode monolithically integrated with a sampled grating distributed Bragg reflector (SGDBR). The phase control sections are properly inserted between the grating bursts of the SGDBR and SGDFB sections for the discrete and continuous tuning. To confirm the feasibility of the new structure, the split-step time domain model is used. The simulation result for a particular design shows that the tuning range as wide as 27 nm is possible with side-mode suppression ratio exceeding 35 dB. Furthermore, the output power is larger than that from SGDBR laser diodes with similar parameters.